chore(Javascript): add factorial recursion (#756)

algorithms/JavaScript/README.md

@@ -40,3 +40,7 @@
 ## Maths
 
 - [Fibonacci Series](src/maths/fibonacci-series.js)
+
+## Recursion
+
+- [Factorial](src/recursion/factorial.js)

algorithms/JavaScript/src/index.js

@@ -7,6 +7,9 @@ require('./linked-lists/singly');
 // Maths
 require('./maths/fibonacci-series');
 
+// Recursion
+require('./recursion/factorial');
+
 // Searching
 require('./searching/binary-search-recursive');
 require('./searching/binary-search');

algorithms/JavaScript/src/recursion/factorial.js

@@ -0,0 +1,17 @@
+// algorithm to calculate factorial of positive integer n
+// Time complexity: O(n)
+
+function factorial(n) {
+  // Error handling
+  // if n is negative or not an integer return string with error message
+  if (n < 0) return 'Only positive numbers allowed';
+  if (!Number.isInteger(n)) return 'Only integers allowed';
+  // if n is a positive integer
+  // base case: we reach 0, return 1
+  if (n === 0) return 1;
+  // until we reach 0 we keep multiplying n by the factorial of n - 1
+  return n * factorial(n - 1);
+}
+
+console.log(factorial(5));
+// output: 120